Stable laser output, both in intensity and wavelength, is desirable in many applications. Laser optical scanning systems, for example, generally allow intensity fluctuation of 1% or less to avoid visible irregularities in images. In videodisk systems, mode hopping causes variations in the location of data written to the optical disk because dispersion causes variations in beam direction and reduces the signal-to-noise ratio, which degrades the quality of the picture derived from the disk. In laser raster printing systems, objectionable artifacts, such as streaks and spots, due to unwanted variations in laser energy delivered to photosensitive media can significantly deteriorate the image quality.
Optical memory devices typically use laser diodes for reading and writing digital data. Back talk noise induced by the resonance phenomenon between the laser diode and the surface of the memory medium can reduce the signal-to-noise ratio of the light output, especially at low output level. The requirements for low-noise laser output may also be very stringent for biomedical imaging and other applications. For example, the tolerance for dynamic intensity fluctuations of a computed radiography scanner may be less than fractions of 1%. In telecommunications, the switching from one mode to another affects the maximum data transmission rate. In laser display, inspection, and lithographic systems, speckle is a common cause to affect image resolution. When used as seed lasers or pump sources for laser diode pumped solid-state lasers (generally referred as DPSS lasers) or optical parametric oscillators/amplifiers (OPO/OPA) or fiber amplifiers/lasers, laser diodes may transfer their optical noise, whether due to intensity or wavelength, to the target laser, causing population inversion fluctuation. Stable intensity and wavelength are also required for harmonic generation and sum/difference frequency generation processes.
Output stability of laser diodes in both intensity and wavelength can be affected by a number of parameters such as operation temperature, driving current, optical feedback, and aging.
Laser diode operating temperature variations can cause laser intensity and wavelength fluctuation because, for a typical laser diode, the mode wavelengths drift with temperature at about 0.06 nm/° C., while the gain peak wavelength shifts at about 0.25 nm/° C. As a result, laser hops from one mode to another. Optical noise thus induced can be as high as several hundred percent of the total signal. Mode hop and mode partition occur randomly among competing modes when their dominance alters. In order to keep laser diode in its stable operation range, a thermoelectric controller, typically consisting of a temperature monitoring section and a temperature feedback control section, is commonly used. For instance, an automatic temperature control system with the accuracy of ±0.5° C. is disclosed in U.S. Pat. No. 6,590,912. This conventional control method, however, has only limited effects on stabilizing laser optical power due to, e.g., ineffectiveness to fast fluctuations and changes in characteristics of some components associated with laser diode aging. In addition, thermoelectric controller is complex and relatively expensive.
Fluctuations in driving current may also cause laser operation unstable in respect of both optical power and wavelength. One way to stabilize laser output power is the use of automatic power control. Such power supply systems typically utilize photoelectric-conversion devices such as photodiodes for detecting laser output and generating feedback signal to the laser diode driver. With this method, the average power of light emitted from the laser diode equals the predetermined value. The limitation of this method includes ineffectiveness in prevention of laser diode from mode hopping or mode partitioning and possible artifacts introduced by parameter shifts with age. Additional artifacts may be induced when back facet photodiodes are utilized for detecting laser output power, which is, unfortunately, the conventional sensing method at present. Coatings for the back mirror having a high reflectivity consists of Bragg stacks, which are pairs of layers with high and low refractive index. The back facet transmittance depends on the match between the quarter-wave length and the thickness of each layer for constructive interference in the stacks. It is generally susceptible to operational conditions such as temperature and diode injection current.
Another unstable factor is optical feedback, that is, any unwanted light reflected back into a laser system by beam forming/shaping optics, laser crystals, or any other optical elements external to the laser. Optical feedback is a persistent problem for laser diodes. Because of their optical characteristics and small size, laser diodes tend to produce a divergent cone of light. In order to produce a well-collimated beam, additional optics is needed, which will inherently cause reflections and feedback. Optical feedback can disturb laser diode operation mode via depopulation of certain lasing levels of the gain medium and change of the gain threshold, deteriorate coherence, and cause fluctuations in intensity and wavelength. The interference between the feedback light and the cavity primary light may split the laser emission spectrum. Optical feedback may also deteriorate the linear relationship between the drive current and laser output, which has an impact to the automatic power control and even causes parasitical oscillation. The latter occurs when the phase of feedback is inverted, a phenomenon called laser diode kinking.
Aging effects include characteristic changes of hardware components and/or optical elements due to degradation and contamination. Over the operating life, some parameters may shift, causing control components, e.g. thermoelectric controllers, ineffective. Changes in reflectance and/or light scattering from particulates may exacerbate optical feedback.
In a DPSS laser, optical noise of the pump diodes may cause instability of the solid-state laser through fluctuations in the pumping intensity and/or wavelength, as well as optical feedback. This is particularly true for a laser gain medium with a narrow absorption band: a shift in diode output wavelength, as a result of, e.g., mode hop, may lead to spectral mismatching of the pumping source with the gain medium.
These instability problems cannot be completely solved by means of conventional automatic temperature feedback control and optical power feedback close loop control or laser drive current control. In fact, neither operation at constant injection current, constant temperature nor constant optical output power suffices to avoid mode hoping, mode partitioning, and unwanted optical feedback.
One approach to stabilization of laser output relies on single longitudinal mode (SLM) operation. In order to maintain laser diode in SLM and mode-hop free operation, a conventional method is based on phase coherence through, e.g., an external laser cavity in conjunction with wavelength-selective feedback from grating or externally coupled cavity. Such laser diode systems usually have relatively large sizes, high costs, less robustness, and require of sophisticated laser current and temperature control, which are not suitable to many original equipment manufacture (OEM) customers. As a matter of fact, highly coherent light sources in many applications are not necessary or even not preferred. Another inherent drawback of the method relates to its ineffectiveness when the pass band of the wavelength selection element is broader than the interval between two adjacent wavelengths of the laser diode Fabry-Perot modes.
A more attractive strategy is based on the opposite philosophy, i.e., intentional decrease of the laser coherence, so that phase related optical noises can be washed out. With this method, the drive current changes rapidly and constantly in such a way that there is no particular longitudinal mode preferable. In other words, the laser operates in a multimode spectrum, normally containing a large number of longitudinal modes. Although the intensity of each individual mode fluctuates all the time, the averaged output essentially keeps unchanged and the overall optical noise decreases significantly.
There have been a number of attempts at controlling laser drive current based on high frequency, e.g. RF, modulation. In such control systems, the drive current is loaded to the high frequency signal generated by a local oscillator so that the superposed current periodically crosses the threshold level. Below this level, the laser diode is off. When the current exceeds the threshold, it turns on the laser. With repeated on-off operation at high frequency, the laser operates in multiple modes because there is not enough time for the completion of mode competition. As a consequence, the signal-to-noise ratio at low laser output can be improved. In fact, there have been several investigations on the application of RF stabilization to optical data reading/writing systems. Exemplary disclosures can be found in U.S. Pat. Nos. 5,175,722; 5,197,059; 5,386,409; 5,495,464; and in particular U.S. Pat. No. 6,049,073. In the last reference, laser output of approximately 20 to 100 mW has been obtained with the use of RF injection. Unfortunately, due to the sine waveform of the RF drive current, this type of stabilization schemes allows only 50% duty cycle. It is not suitable to high power lasers because this would overdrive the laser and decrease its lifetime. In extreme cases, there is a possibility for the power supply to back-bias the laser diode and even destroy it.
In an attempt to extend the above-discussed RF stabilization scheme to high power region, Roddy and Markis, in U.S. Patent Application Publication Nos. 2003/0128725 and 2002/0125406 and U.S. Pat. Nos. 6,625,381 and 6,999,838, have invented a control system, which allows a laser diode to operate predominantly above the threshold. Specifically, the injection circuit generates a radio frequency waveform, which provides a drive current that varies between the point slightly below a lasing threshold and a level above a DC bias point. Since the drive current is asymmetric about the DC bias, a duty cycle, which is greater than 50%, can be achieved. Therefore, a high average laser output power can be obtained without the risk of exceeding the maximum rated current, Imax. Unfortunately, the RF waveforms generated from their inventive system provided for such a modulation degree that may be ineffective to certain devices. These laser devices include blue or violet diodes, which require the drive current drops far below the threshold to completely turn off the laser and eliminate the memory. More importantly, the injection circuits in their inventive system are impractical due to the incapability of responding to RF signals with the desired performance. In particular, due to the limitations of the electronic components, the shunt modulator circuit disclosed in their patents may not be able to rapidly turn the laser diode on and off as anticipated. Another shortcoming of their invention was related to the use of back facet photodiode for monitoring laser output. Fluctuations in the temperature and/or drive current may introduce additional noise through the temperature/wavelength-dependent back facet mirror transmittance and the automatic power control loop. Moreover, the system disclosed in their patents was subject to undesirable optical feedback.
Stabilization of laser sources for pumping solid-state lasers or fiber amplifies/lasers was investigated by Ziari et al. in U.S. Pat. No. 6,215,809. By the use of a dither circuit, coherence collapse is achieved and the laser source is repeatedly perturbed from one operating mode to another at a rate that is too high for the gain element to response. In the U.S. Pat. No. 6,240,119 issued to Ventrudo, kink-free operation was achieved by repetitive switching between the states of coherence and coherence collapse through variations of the drive current amplitude at a rate considerably higher than the reciprocal of the relaxation time of the excited state of the optical gain medium, which is typically from several microseconds to milliseconds.
Application of RF modulation to second harmonic generation (SHG) is described in, e.g., U.S. Pat. No. 6,678,306. Dependence of the SHG efficiency on the modulation frequency and degree were investigated. Because the degree of modulation adopted by Sonoda was rather deep, the bandwidth of the primary laser spectrum was wider than the phase matching tolerance. Consequently, an external oscillator including a narrowband filter for wavelength selection was used in order to realize the quasi-phase matching of the SHG. In another work, as disclosed in U.S. Pat. No. 6,385,219, Sonoda has investigated DPSS laser stabilized by RF modulation.
In spite of these efforts, a large room for further improvement of laser stabilization still remains to be filled. In particular, the stabilization methods of the prior art are subject to variations in the environmental conditions and are not applicable to short wavelength diode lasers such as violet or blue laser diodes or diode modules. The successes in stabilizing DPSS lasers and fiber lasers are very limited. To date, the RF modulation, as described in the prior art, produces only broadband, multimode laser output. Because of this limitation, the prior art has not been very successful in applications requiring stable narrowband or single longitudinal mode laser sources. Examples of such applications include high-order harmonic generation and Raman scattering. In addition, no attempts have been made to stabilize slave lasers in the absence of complex cavity length control and phase locking schemes. Moreover, the prior art has not disclosed the critical role of appropriately selecting the laser operation parameters. As a result, optical noise associated with mode hop and/or mode partition may still occur. Our invention advantageously addresses these deficiencies and enables relatively compact and low-cost high power solid-state lasers, which can be operated stably and reliably at various wavelengths, from near infrared to the entire range of visible, in single or multi mode.